Funding provided by the Los Alamos National Laboratory Directed Research and Development Program

Transcription

1 Combining ferroelectricity and magnetism: the low energy electrodynamics Diyar Talbayev Center for Integrated Nanotechnologies Los Alamos National Laboratory

2 Acknowledgements Los Alamos National Laboratory Toni Taylor A.V. Balatsky Darryl Smith Stuart Trugman John O Hara Boston University Rick Averitt Rutgers University Seongsu Lee S.-W. Cheong Osaka University, Japan Tsuyoshi Kimura Inha University, Korea Namjung Hur Nanyang Technological University, Singapore Elbert Chia UCSD Andrew Laforge Dmitri Basov Funding provided by the Los Alamos National Laboratory Directed Research and Development Program

3 Ferroic properties FERROMAGNETISM High-power current-driven write operation FERROELECTRICITY Low-power voltage-driven write But Issues of fatigue need to be overcome enter MULTIFERROICITY!!!

4 Magnetoelectric multiferroics MULTIFERROICS materials with coexisting magnetism and ferroelectricity. Magnetic order Electric polarization magnetoelectric interaction Possible applications for magnetoelectric materials Magnetoelectric memory Magnetically switched electro-optic device Electric-field-modulated visible Faraday rotator Magnetically (electrically)-modulated piezoelectric (piezomagnetic) devices, etc. V. E. Wood, A. F. Austin, Int. J. Magnetism 5, (1974)

5 Magnetoelectric effect BiFeO 3 : ferroelectric antiferromagnet Fe ions: O Bi Fe Apply electric field E Turn the magnetic moments! Top view Y.-H. Chu et al., Nat. Materials (2008) easy axis for magnetization

6 Dynamic magnetoelectric effect BiFeO 3 Bi Top view Rock the polarization P M O Fe excite phonons Magnetic moments respond! Y.-H. Chu et al., Mixing of magnetic and lattice vibrations DYNAMIC MAGNETOELECTRIC EFFECT Nat. Materials (2008)

7 Magnetic and lattice motion interacts with light Electromagnetic wave incident on the crystal?? Phonons ~ oscillating electric dipole Coupling to the electric field of the light wave Phonons in BiFeO 3 Lobo et al., Phys. Rev. B (2007) 33 cm -1 ~ 1 THz ~ 4 mev Resonance in the dielectric function, a.k.a. optical conductivity: ε(ω) σ(ω)

8 Magnetic and lattice motion interacts with light Electromagnetic wave incident on the crystal Magnons ~ oscillating magnetic dipole 0.8 Magnetic excitations in BiFeO 3 33 cm -1 ~ 1 THz ~ 4 mev Coupling to the magnetic field of the light wave Transmission T=30 K Frequency (cm -1 ) D. Talbayev et al., unpublished Resonance in the magnetic susceptibility: χ(ω) µ(ω)

9 Mix it up: electromagnons TbMn 2 O 5 : antiferromagnet and ferroelectric Sushkov et al, Phys. Rev. Lett (2007) electric dipole active magnon : electromagnon Similar observations : multiferroics TbMnO 3, Eu 0.75 Y 0.25 MnO 3 Pimenov et al., Nature Phys (2006) Valdes Aguilar et al., Phys. Rev. B R (2007) Pimenov et al., Phys. Rev. B (2008)

10 Electromagnons determine magnetoelectric functionality TbMn 2 O 5 : antiferromagnet and ferroelectric Sushkov et al, Phys. Rev. Lett (2007) onset of electromagnon response antiferromagnet, ferroelectric paramagnet (i) (ii) Electromagnon contributes to static dielectric constant: Electromagnon properties depend on the microscopic magnetoelectric coupling ε σ1( ω ) 1( 0 ) = 1+ 8 dω 2 ω 0 i) Properties of electromagnons

11 Dynamic magnetoelectric effect in hexagonal Ba Sr 1.4 Zn Zn 2 Fe Fe 12 O 22 Layered magnetic structure Several magnetic phases N. Momozawa and Y. Yamaguchi, J. Phys. Soc. Jpn (1993) T. Kimura, G. Lawes, and A.P. Ramirez, PRL (2005)

12 Dynamic magnetoelectric effect in hexagonal Ba Sr 1.4 Zn Zn 2 Fe Fe 12 O 22 Control of electric polarization by magnetic field: T. Kimura, G. Lawes, and A.P. Ramirez, PRL (2005)

13 Time-domain studies of elementary excitations visible optical pulse excitation, a.k.a. pump Pump and probe wavelength: 800 nm (~1.5 ev) Electrons, phonons, magnons optical pulse interrogation (probe) of material s response Dynamics of photo-excited quasiparticles often exposes properties not detected by conventional probes transport, magnetization, or optical conductivity

14 Time-domain detection of magnetic motion Magneto-optical Kerr effect (MOKE) rotation of polarization of light upon reflection by a magnetized medium reflected probe beam ferromagnet La 2/3 Ca 1/3 MnO 3 MOKE intensity M arrival of Time (ps) pump M & = γm H eff H H + H + eff = 0 anisotropy H demag D. Talbayev et al, Phys. Rev. B (2006); Appl. Phys. Lett (2005)

15 Time-resolved reflectance : coherent response probe pump Ba 0.6 Sr 1.4 Zn c axis B Zn 2 Fe R/R (x10-6 ) Fe 12 O : photoinduced change in reflectance 0 T 0.1 T 0.5 T 1.0 T Coherent excitation of magnetic or lattice motion T=10 K 400-nm pump and probe 1.5 T 2.0 T 2.5 T Time (ps) D. Talbayev et al., Phys. Rev. Lett (2008)

16 Magnetic or lattice? ocsillation frequency ω N. Momozawa and Y. Yamaguchi, J. Phys. Soc. Jpn (1993) Os scillation frequency (GHz) field normal to the c axis field along the c axis Magnetic field (T) T=10 K Strong evidence in support of magnetic origin of the excitation electron spin resonance, i.e., k=0 magnon D. Talbayev et al., Phys. Rev. Lett (2008)

17 More evidence against the lattice LuMnO 3 Oscillation frequency: f PHONONS n / λ R/R (arb. units) 400 nm 800 nm T=15 K H=0.2 T ( R/R(800)-decay) x 10 D. Lim et al., Appl. Phys. Lett (2003) Time (ps) Identical oscillation frequency at different probe wavelengths D. Talbayev et al., Phys. Rev. Lett (2008)

18 Calculation of magnon frequencies B H = H ex ( M Si M Lj ; M M ; M M 2 ) + D ( M Liz + M 2 L1 L2 S1 S 2 Siz ) D the only free parameter T. Kimura, G. Lawes, and A.P. Ramirez, PRL (2005) N. Momozawa and Y. Yamaguchi,J. Phys. Soc. Jpn (1993)

19 Calculation of magnon frequencies D. Talbayev et al., Phys. Rev. Lett (2008)

20 Dynamic magnetoelectric effect R/R (x10-6 ) Ba 0.6 Sr 1.4 Zn Zn 2 Fe Fe 12 O 22 T=10 K 400-nm pump and probe 0 T 0.5 T 1.5 T Time (ps) Modulation of reflectance n 1 R = n + 1 n = ε Modulation of n and ε by magnon motion: dynamic magnetoelectric effect 2 Optical detection of magnetic state using reflection implications for data storage and spintronics.

21 Magnetoelectricity in hexagonal HoMnO 3 Ferroelectric P FE Triangular antiferromagnet, T N =72 K T c =875 K Mn ions: z=c/2 z=0 40 K < T < 72 K T R T N z=c/2 5 K < T < 40 K T Ho T R z=0 T < 5 K T Ho Litvinchuk et al., JPCM (2004) Vajk et al., PRL (2005)

22 Magnetoelectricity in hexagonal HoMnO 3 Magnetism of Ho 3+ (S=2, L=6, J=8) ions: Two sites: Ho(1) C 3v Ho(2) C 3 ordered for T<T Ho =5 K z T<T N E=0 Applied electric field induces magnetization of Ho ions: H int =ΣS Ho ΑS Mn Magnetization n Lottermoser et al., Nature (2004)

23 Far-infrared study of magnetic excitations HoMnO 3 h c (001) (110) H static field lightwave H h

24 Far-infrared study of magnetic excitations 1 THz 300 µm ev 33cm K excitation pulse E field amplitude SAMPLE REFERENCE gating pulse Time (ps) 25 Broadband Measurement of electric field E (1) Photoconductive antennas (2) Electro-optic crystals E field amplitude SAMPLE REFERENCE Frequency (THz)

25 Magnetic excitations in HoMnO 3 h T=10 K h Transmission h c 0.05 h c Frequency (cm -1 ) Crystal field excitations of Ho ions Antiferromagnetic resonance (AFMR) of Mn ions Neutron scattering: Vajk et al., Phys. Rev. Lett (2005)

26 Crystal field splitting of Ho 3+ ground state Two Ho 3+ (S=2, L=6, J=8) cites with C 3 and C 3v point symmetries m = ±8 ±7 J=8 ±1 0 ± 8,± 2, m4 6 s,0 6a 6 s,0 3a = 3 3 3s = The electrostatic environment of Ho ions determines the crystal field splitting ± 7,± 1, m5 Abragam and Bleaney, Electron paramagnetic resonance of transition ions, Clarendon press, 1970

27 Magnetic resonance of Mn ions z M 2 B M 1 λ M 3 B Classical treatment F = λσm i M j + K ( M z i 2 ) BΣM z i Interaction parameters measured by neutron scattering in zero field by Vajk et al., Phys. Rev. Lett (2005)

28 Magnetic resonance of Mn ions T=10 K, B c T=10 K, B c Frequency (cm -1 ) Magnetic field (T) Magnetic field (T) D. Talbayev et al., Phys. Rev. Lett (2008)

29 Magnetic resonance of Mn ions H ex = λm 0 H d = KM 0 Palme et al., Solid State Comm (1990) Why the discrepancy? D. Talbayev et al., Phys. Rev. Lett (2008)

30 Exchange coupling between Ho and Mn ions YMnO 3 : similar material hexagonal lattice, ferroelectric, triangular antiferromagnet but Y ions are not magnetic and the calculation works!! Penney et al., J. Appl. Phys (1969) Sato et al., Phys. Rev. B (2003) Discrepancy in HoMnO 3 is due to Ho-Mn (HM) exchange interaction: F HM = H 6J zχ g gµ z HM ex 2 B = J ~ ij S Ho iz S Mn jz Mn B M iz = λ χbχ B g µ Mn z Effective magnetic field acting on Mn ions! = HM J z B z M B S Mn iz λ HM = 1 D. Talbayev et al., Phys. Rev. Lett (2008)

31 Magnetoelectricity in HoMnO 3 Ferromagnetic exchange between Ho and Mn: H HM ex ~ ~ z = J S S < 0 z ij Ho iz Mn jz Same interaction responsible for the magnetoelectricity: J ij Lottermoser et al., Nature (2004) T<T N E Magnet tization Magnons allow the determination of microscopic details of magnetoelectric interaction. In HoMnO 3, it is the Ho-Mn magnetic exchange coupling.

32 Summary (i) (ii) Magnetic and lattice excitations (magnons and phonons) play a fundamental role in the quest for understanding and exploiting magnetoelectric materials Detection of magnetic motion and magnetic state via the modulation of reflectance R/R (x10-6 ) Ba 0.6 Sr 1.4 Zn T Zn 2 Fe T=10 K 400-nm pump and probe Fe 12 O Time (ps) (iii) Properties of magnons and phonons help reveal the microscopic details of magnetoelectric interaction H HM ex = ~ J HoMnO 3 z ij S Ho iz S Mn jz